System and method for using a solar cell in wireless communication

ABSTRACT

An energy-harvesting system capable of wireless communication is disclosed which includes a solar cell adapted to generate charge from incident light on the solar cell; an energy-harvesting circuit coupled to the solar cell, the energy-harvesting circuit adapted to convey charge from the solar cell to an energy reservoir; a data communication circuit adapted to encode data to be communicated by the solar cell by adjusting the voltage across the solar cell into at least two states including 1) an open circuit (OC) state, where the current through the solar cell is substantially zero, and 2) a maximum power point (MPP) state, where power (V×I) drawn from the solar cell is maximum, wherein the solar cell in response to the changes in its voltage and the incident light emits luminescent radiation with an intensity corresponding to the at least two states, thereby forming a wireless transmitter.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present patent application is related to and claims the prioritybenefit of U.S. Provisional Patent Application Ser. No. 62/727,315 filedSep. 5, 2018, the contents of each of which are hereby incorporated byreference in its entirety into the present disclosure.

STATEMENT REGARDING GOVERNMENT SUPPORT

The present disclosure was not supported by the US government.

TECHNICAL FIELD

The present disclosure generally relates to solar cells, and inparticular to the modulation of photo-luminescent andelectro-luminescent emissions of said solar cells.

BACKGROUND

This section introduces aspects that may help facilitate a betterunderstanding of the disclosure. Accordingly, these statements are to beread in this light and are not to be understood as admissions about whatis or is not prior art.

Many technologies across a plurality of applications (i.e.Internet-of-Things (IoT) applications and devices, manufacturingequipment, shipping technologies, etc.) support some form of wirelesscommunication. One of the most common methods is through the emittingand receiving of radio signals. These technologies use devices that emitradio signals to transmit encoded data which is then read by anotherdevice in order to determine what corresponding set of actions or eventsmust take place. One such technology commonly used is Radio FrequencyIdentification (RFID). However, RFID technology has limitations makingit inconvenient to use in certain situations. RFID devices are energizedby the radiation emitted by a reader, requiring the RFID device to be inclose proximity to the reader.

For this type of communication, radio-based communication has been thedominant technology for establishing wireless connectivity in IoTapplications. Most existing solutions rely on unlicensed radio bands forease of deployment and adoption. However, these bands are expected tobecome increasingly crowded as more IoT devices are deployed resultingin higher interference levels and slower throughputs.

Furthermore, RFID tags that are mounted on metallic surfaces requirespecial mounting equipment in order to avoid detuning resulting in anincreased cost to the end user and requiring a larger area to be used.RFID devices are also very limited in underwater usage. Low-FrequencyRFID devices are capable of working underwater, however such RFIDdevices have a greatly diminished range.

Therefore, there is an unmet need for a novel approach to providewireless communication of data between two devices without relying oncrowded radio-frequency bands.

SUMMARY

An energy-harvesting system capable of wireless communication isdisclosed. The system includes a solar cell adapted to generate chargefrom light incident on the solar cell. The system also includes anenergy-harvesting circuit coupled to the solar cell, theenergy-harvesting circuit adapted to convey charge from the solar cellto an energy reservoir. The system further includes a data communicationcircuit adapted to encode data to be communicated by the solar cell byadjusting voltage across the solar cell into at least two statesincluding an open circuit (OC) state, where the current through thesolar cell is substantially zero; and 2) a maximum power point (MPP)state, where power (V×I) drawn from the solar cell is maximum. The solarcell in response to a change in its voltage and the incident light emitsluminescent radiation with an intensity corresponding to the at leasttwo states, thereby forming a wireless transmitter.

A method of harvesting energy while providing wireless communication isalso disclosed. The method includes receiving light by a solar celladapted to generate charge from the light incident on the solar cell.The method also includes harvesting energy from the solar cell by anenergy-harvesting circuit coupled to the solar cell. In addition, themethod includes conveying charge from the solar cell to an energyreservoir by the energy-harvesting circuit. Furthermore, the methodincludes adjusting electrical voltage across the solar cell into atleast two states including 1) an open circuit (OC) state, where thecurrent through the solar cell is substantially zero; and 2) a maximumpower point (MPP) state, where power (V×I) drawn from the solar cell ismaximum. The solar cell in response to a change in its voltage and theincident light emits luminescence with an intensity corresponding to theat least two states, thereby forming a wireless transmitter. The methodfurther includes wirelessly communicating data based on the at least twostates.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a single-diode equivalent circuit of a solar cell includingemitted luminescent radiation.

FIG. 2 is a graphical representation of the relationship between theluminescent radiant flux emitted by a solar cell and the voltage acrossthe solar cell.

FIG. 3 is a depiction of an experimental setup used to verify therelationship depicted in FIG. 2.

FIG. 4 is a graphical representation of the measured relationshipbetween the luminescent radiant flux emitted by the solar cell of FIG. 3and voltage across the solar cell.

FIG. 5 is a conceptual diagram of an embodiment of an OFID communicationsystem having an active reader.

FIG. 6 is a conceptual diagram of an embodiment of an OFID communicationsystem having a passive reader.

FIG. 7A is a system-level diagram of proof-of-concept circuit for a PLmodulator and EH circuit.

FIG. 7B shows the basic PL modulation principle employed in FIG. 7A.

FIG. 7C is a plot of the expected voltage across the solar cell of FIG.7A and the PL radiant flux as a function of resistance.

FIG. 8 is a schematic diagram of an embodiment of a proposed PLmodulator and EH circuit.

FIGS. 9A-9D are various circuit diagrams used to provide solutions forvariables of the proposed PL modulator and EH circuit shown in FIG. 8.

FIG. 9E is a flow chart showing a process to provide solution forvariables of the proposed PL modulator and EH circuit shown in FIG. 8.

FIG. 9F is a plot of the average output voltage of a DC-DC converteremployed in FIG. 8 as a function of duty cycle.

FIG. 10A is a plot of the relationship between input impedance of theDC-DC converter employed in FIG. 8 and the duty cycle of the clocksignal.

FIG. 10B is a plot of the relationship between luminescent radiant fluxof a solar cell employed in FIG. 8 and the duty cycle of the DC-DCconverter.

FIG. 11A is a schematic diagram of an experimental setup employed totest the functionality of the circuit proposed in FIG. 8.

FIGS. 11B-11C and recorded waveforms of the different tests performedbased on the circuit provided in FIG. 11A.

FIG. 11D is a graph of voltage vs. time representing output of an 8^(th)order low-pass filter.

FIG. 11E is a collection of graphs of voltage vs. time for recordedwaveforms for an on-off keying modulation for three frequencies: 1 kHz(top row), 5 kHz (middle row) and 10 kHz (bottom row).

FIG. 12 is a schematic diagram of a PL modulator circuit, according toone embodiment of the present disclosure.

FIG. 13 is a schematic diagram of a PL modulator circuit, according toone embodiment of the present disclosure.

FIG. 14 is a schematic diagram of a PL modulator circuit, according toone embodiment of the present disclosure.

FIG. 15 is a schematic diagram of a PL modulator circuit, according toone embodiment of the present disclosure.

FIG. 16 is a schematic diagram of a PL modulator circuit, according toone embodiment of the present disclosure.

FIG. 17 is a schematic diagram of a PL modulator circuit, according toone embodiment of the present disclosure.

FIG. 18 is a schematic diagram of a PL modulator circuit, according toone embodiment of the present disclosure.

FIG. 19 is a schematic diagram of a PL modulator circuit, according toone embodiment of the present disclosure.

FIGS. 20 and 21 are schematics of two embodiments of the PL modulationaccording to the present disclosure including tandem solar cells.

FIG. 22 is an embodiment of the EL modulation circuit, according to thepresent disclosure.

FIG. 23 is an embodiment of the EL modulation circuit, according to thepresent disclosure.

FIG. 24 is an embodiment of the EL modulation circuit, according to thepresent disclosure.

FIG. 25 is an embodiment of the EL modulation circuit, according to thepresent disclosure.

FIG. 26 is an embodiment of the EL modulation circuit, according to thepresent disclosure.

FIG. 27 is an embodiment of the EL modulation circuit, according to thepresent disclosure.

FIG. 28 is a prior art schematic diagram for a solar cell energyharvesting.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of thepresent disclosure, reference will now be made to the embodimentsillustrated in the drawings, and specific language will be used todescribe the same. It will nevertheless be understood that no limitationof the scope of this disclosure is thereby intended.

In the present disclosure, the term “about” can allow for a degree ofvariability in a value or range, for example, within 10%, within 5%, orwithin 1% of a stated value or of a stated limit of a range.

In the present disclosure, the term “substantially” can allow for adegree of variability in a value or range, for example, within 90%,within 95%, or within 99% of a stated value or of a stated limit of arange.

A novel approach to a wireless optical communication scheme thatmodulates photo-luminescent (PL) and electro-luminescence (EL) emissionsof high-efficiency solar cells to transmit information wirelessly ispresented. The wireless transmission of information that the presentdisclosure is concerned with is made possible by the fact thathigh-efficiency solar cells are also good emitters of light. Theemissions of light, referred to as luminescent emissions, are a functionof voltage across the cell and can be modulated in order to transmitinformation. Devices of this type of communication will be referred toas Optical Frequency Identification (OFID) devices.

Solar cells are adapted to convert incident light to electrical chargethat can be stored in a charge reservoir. Referring to FIG. 28, aschematic of a prior art system coupled to a solar cell is shown. Thesystem includes a solar cell which provides charge in minute amounts.The charge is transferred to a boost circuit, shown as the powerconverter, which is a clocked circuit and is responsible to transfer theminute amounts of charge on a switched basis to the energy reservoir.The input clock to the power converter is modulated based on informationfrom the solar cell and the power converter circuit. The power convertercircuit can be a boost DC-DC converter, a buck-boost DC-DC converter, ora buck DC-DC converter, as these types of circuits are known to a personhaving ordinary skill in the art and the description of the same is thusassumed to be known. In addition, the power converter circuit can be abi-directional DC-DC converter, that can be used in the EL modality.

Solar cells are advantageously capable of not only harvesting incidentenergy, but also transmitting and receiving information. As shown inFIG. 28, solar cells are used to convert radiant (light) energy intoelectrical energy which is typically intended to power some device orstore the energy in some form of capacitor or battery. Solar cells arealso capable of receiving information which has been encoded opticallyby virtue of their photo-transduction property. Furthermore,high-efficiency solar cells, are advantageously good emitters of light,which as discussed above are referred to as luminescent emissions. Forinstance, gallium arsenide (GaAs) solar cells, have strong luminescentemissions in the near infrared portion of the electro-magnetic spectrum.Moreover, the intensity of the luminescent emissions from a solar cellis a function of the cell's voltage.

There are two approaches to cause wireless communication based onluminescent emissions: 1) photo-luminescence (PL) which is directed toluminescence generated by incident light and which can be modulated, or2) electro-luminescence (EL) which is directed to luminescence generatedby electrical energy and which can also be modulated. Thus, in the PLapproach, incident light is required to communicate wirelessly. However,in the EL approach, incident light is not required and thus the solarcell can still communicate in complete darkness.

Referring to FIG. 1, a single-diode equivalent circuit 102 of a solarcell 100 is shown. The single-diode equivalent circuit 102 includesemitted luminescent radiation flux Φ_(lum) measured in Watts, where theincident radiant flux of the solar cell 100 is denoted by Φ_(in). In thesingle-diode equivalent circuit 102, the diode modeling the “knee” inthe current-vs-voltage (IV) curve of a solar cell, known to a personhaving ordinary skill in the art, is replaced by a light-emitting diode104 (LED). Luminescence is the emission of light from certain materials,such as semiconductors, under external excitation, and which is notcaused by an increase in temperature. Photo-luminescence (PL) occurswhen the external excitation is provided by light, whereaselectro-luminescence (EL) occurs when the external excitation isprovided by injected electrical charges. In semiconductors where thereis a conduction band and a valence band separated by an energy band gap,luminescence occurs when electrons from the conduction band transitionto the valence band emitting a photon of energy E_(g) equal to theelectron's excess energy. The energy of a photon can be quantified byE _(ph) ^(e) =hc/λ _(e)  (1)where h is Planck's constant,c is the speed of light, andλ_(e) is the wavelength. By equating equation (1) to the emitted photonenergy E_(g), the wavelength of the emitted photons can be calculated.For instance, gallium arsenide (GaAs) has a band gap energy of 1.4 eVresulting in an emitted wavelength of 886.3 nm.

With further reference to FIG. 1, other parameters are defined as

R_(sh) represents the shunt resistance,

C_(j) represents the junction capacitance,

R_(sr) represents the series resistance of the solar cell 100, and

I_(ph) represents the photo-generated current of the solar cell 100. Formonochromatic light, the value of I_(ph) is given by the formula:

$\begin{matrix}{I_{ph} = {{q\left( {1 - R} \right)}{{QE}\left( \lambda_{i} \right)}\left( \frac{\Phi_{in}}{E_{ph}^{i}} \right)}} & (2)\end{matrix}$where,

$\left( \frac{\Phi_{in}}{E_{ph}^{i}} \right)$is ϕ_(n) which is the incident radiant fluxq is the electron's charge,R is the fraction of the photo-generated electrons that recombine insidethe solar cell 100 and do not contribute to the current,QE(λ_(i)) is the quantum efficiency at the wavelength of the incidentlight λ_(i), andE_(ph) ^(i) is the energy of an incident photon. The current through theLED 104 I_(d) is related to the implied voltage V_(d) by the equation:I _(d) =I _(s)(e ^(V) ^(d) ^(/nV) ^(T) −1)  (3)where, n is the ideality factor of the diode,V_(T) is the thermal voltage, and I_(s) is the reverse saturationcurrent. The luminescent radiant flux Φ_(lum) emitted by the solar cell100 has two components represented by the equation:

$\begin{matrix}{\Phi_{lum} = {\underset{{first}\mspace{14mu}{component}}{\underset{︸}{{\eta\kappa}\;{{RQE}\left( \lambda_{i} \right)}\Phi_{in}}} + \underset{\underset{{second}\mspace{14mu}{component}}{︸}}{{\eta\kappa}\;{{RE}_{ph}^{e}\left( \frac{I_{d}}{q} \right)}}}} & (4)\end{matrix}$where the first component is due to the photo-generated electrons thatrecombine radiatively and the second component is due to the currentI_(d) that flows through the solar cell 102. In (4), η is the fractionof photons generated inside and escape from the solar cell 100 (i.e.photons that do not get internally reflected or re-absorbed), and κ isthe fraction of recombined electrons that recombine radiatively (i.e.generate a photon upon recombination). In situations with no radiantflux (Φ_(in)=0), the emitted luminescent radiant flux is only due tocharges injected into the solar cell 100 via electro-luminescence (EL).

For most solar cells 100, R_(sr) is about 0. By combining equations (3)and (4) and assuming V_(d) is about the same as V_(D), yields thefollowing equation:

$\begin{matrix}{\Phi_{lum} = {{\eta\kappa}\;{R\left( {{{{QE}\left( \lambda_{i} \right)}\Phi_{in}} - \frac{E_{ph}^{e}I_{s}}{q} + \frac{E_{ph}^{e}I_{s}e^{V_{D}/{nV}_{T}}}{q}} \right)}}} & (5)\end{matrix}$Equation (5) shows an exponential relationship between Φ_(lum) and theexternal voltage V_(D). This relationship is exploited in the presentdisclosure in order to modulate the luminescent emissions of a solarcell to convey information. Of particular interest is the luminescentradiant flux at short circuit (SC) Φ_(lum) ^(sc), open circuit (OC)Φ_(lum) ^(oc) and at the maximum power point (MPP) Φ_(lum) ^(mpp). Fromequation (5) one can obtain the following three equations:

$\begin{matrix}{\Phi_{lum}^{sc} = {{\eta\kappa}\;{R\left( {{QE} \cdot \Phi_{in}} \right)}}} & (6) \\{\Phi_{lum}^{oc} = {{{\eta\kappa}\;{R\left( {{QE} \cdot \Phi_{in}} \right)}} - \frac{E_{ph}^{e}I_{s}}{q} + \frac{E_{ph}^{e}I_{s}e^{V_{oc}/{nV}_{T}}}{q}}} & (7) \\{\Phi_{lum}^{mpp} = {{{\eta\kappa}\;{R\left( {{QE} \cdot \Phi_{in}} \right)}} - \frac{E_{ph}^{e}I_{s}}{q} + \frac{E_{ph}^{e}I_{s}e^{V_{mpp}/{nV}_{T}}}{q}}} & (8)\end{matrix}$where V_(oc) is the solar cell's 100 open circuit voltage and V_(mpp) isthe solar cell's 100 voltage at the maximum power point. Furthermore,considering a large R_(sh), the following explicit solutions for V_(oc)and V_(mpp) can be obtained:

$\begin{matrix}{V_{oc} = {{nV}_{T}{\log\left( \frac{I_{ph} + I_{s}}{I_{s}} \right)}}} & (9) \\{V_{mpp} = {{{nV}_{T} \cdot {W\left( {\frac{I_{ph} + I_{s}}{I_{s}} \cdot e} \right)}} - {nV}_{T}}} & (10)\end{matrix}$where, W (⋅) is the Lamber-W function.

With reference to FIG. 2, a graphical representation of the relationshipbetween the luminescent radiant flux Φ_(lum) and external voltage V_(D)as described by equation (5) for η=0.4, κ=0.8, R=0.1, QE=0.8,I_(s)=3.506×10⁻¹⁷ A, and Φ_(in)=1.5 mW is presented. Values forluminescent radiant flux at short circuit (SC) Φ_(lum) ^(sc), opencircuit (OC) Φ_(lum) ^(oc) and at the maximum power point (MPP) Φ_(lum)^(mpp) are marked by circles on the figure and are given by theappropriate combination of equations (6), (7), (8) and equations (9),and (10). For instance, if one were to obtain the open circuitluminescent radiant flux, a combination of equations (7) and (9) wouldbe used. For cases where the external voltage V_(D) is greater than theopen circuit voltage V_(oc) (V_(D)>V_(oc)), the net current flow is intothe solar cell 102. Hence, for the case where V_(D)>V_(oc), theexcitation that stimulates luminescence includes injected charges. Inthis scenario the solar cell 102 consumes energy instead of generatingenergy. However, high levels of luminescent radiant flux can be achievedfor V_(D)>V_(oc). The region V_(D)<V_(oc) corresponds tophoto-luminescence (PL). In this region, the luminescent radiant fluxΦ_(lum) can be modulated using different strategies. For example, thecell can be switched between OC and MPP or OC and SC for On-Off Keying(OOK) modulation. In the OC state, the PL is at its maximum, while atSC, PL is at its minimum, and further at MPP, the luminescence is inbetween. The discrimination between the three luminescence levels allowsbinary and ternary digital communication modulation schemes. Betweenthese three states, at the maximum power point (MPP) the solar cell 102provides the maximum amount of energy. Therefore, a modulation schemethat includes the MPP may be preferable in terms of energy harvesting.Pulse Amplitude Modulation (PAM) and analog modulation of the PL radiantflux is also possible by operating the solar cell 102 in the regionbetween OC and MPP or between OC and SC.

The exponential relationship between Φ_(lum) and V_(D) was verifiedexperimentally using the setup shown in FIG. 3. In this firstexperimental setup 300, a GaAs solar cell 302 was placed at a distancedi equal to about 6.5 centimeters in front of a red LED flashlight 304with emitted irradiance of 23.98 mW/cm². It should be known that, asdiscussed above, GaAs solar cells 302 have strong luminescent emissionsin the near infrared portion of the electro-magnetic spectrum. Anoptical power sensor 306, with an 850 nm long-pass optical filter infront of it, not shown in the figure, was placed at the same distance difrom the GaAs solar cell 302. Radiant flux emitted by the GaAs solarcell 302 was then recorded. The first experimental setup 300 wasisolated from incident light in order to avoid unintended excitations.

With reference to FIG. 4, a graph comparing the measured power of theGaAs solar cell 302 from the first experimental setup 300 is shown. Thegraph compares two scenarios, one where the GaAs solar cell 302 wasilluminated by the LED flashlight 304 and one where the GaAs solar cell302 was dark, meaning that the LED flashlight 304 was not on. From FIG.4 it can be seen that the exponential relationship between Φ_(lum) andV_(D) given by equation (5) was observed. Visual confirmation of PL andEL emissions were obtained using a night-vision camera sensitive toinfrared light. Four different scenarios were tested for confirmation.The first scenario included the GaAs solar cell 302 illuminated by theLED flashlight 304 and with an open circuit between the GaAs solar cell302 terminals (OC PL). In this scenario the night-vision camera pickedup the infrared light emitted by the GaAs solar cell 302. The secondscenario included the GaAs solar cell 302 illuminated by the LEDflashlight 304 while the GaAs solar cell 302 was biased at the MPP (MPPPL). In this scenario the night-vision camera picked up diminishedinfrared light. The third scenario included the GaAs solar cell 302illuminated by the LED flashlight 304 with a short circuit between theGaAs solar cell's 302 terminals (SC PL). In this scenario thenight-vision camera picked diminished infrared light. The fourthscenario included the GaAs solar cell 302 while the LED flashlight 304was turned off and an external voltage of 1.0 V was applied to the GaAssolar cell 302 (EL). In this scenario the night-vision camera picked upthe EL emission given off by the GaAs solar cell 302.

It can be observed that there is a significant difference between theluminescent radiation emitted by the solar cell at OC and at MPP or SC.This observation qualitatively confirms the model and measurementspreviously discussed. A strong EL emission can also be observedsuggesting that even in a complete dark environment, the GaAs solar cell302 can still be employed to transmit information.

In particular, it can be seen from the graph of FIG. 4 that the shortcircuit (SC) produces 0.12 μW when the voltage across solar cell is 0 V.Conversely, the MPP mode produces 0.25 μW when the voltage across solarcell is 0.92 V. Further in comparison with open circuit, the opencircuit (OC) produces 4.8 μW when the voltage across solar cell is 1.2V.

Referring to FIG. 5 a conceptual embodiment of an OFID communicationsystem 500 is shown. Communication is between an OFID device 502 and areader 504. The reader 504 shown in FIG. 5 is an active reader, that is,a reader 504 that illuminates the solar cell 506 in the OFID device 502.The light generated by the reader 504 carries radiant energy from thereader 504 to the OFID device 502 to energize and activate the OFIDdevice 502. To transmit data to the OFID device 502, the reader 502modulates the power of the generated light. The reader 504 is alsoequipped with a photo-detector 508, amplifiers 5101 and 5102, focusingoptics 512 and an optical filter 514 to detect the luminescent radiationemitted by the solar cell 506. The main components of the OFID deviceare: a data receiver 516 whose function is to demodulate variations inoptical power to recover data sent by the reader 504; a luminescencemodulator 518 which modulates the luminescent emissions (PL or EL) ofthe solar cell 506 to transmit data back to the reader 504 and an energyharvester circuit 520 whose function is to draw out energy from thesolar cell 506 to either power up the OFID device 502 directly or tocharge an energy reservoir 522.

The energy harvester 520 may also boost and stabilize the output voltageof the solar cell 506 in order to provide a supply voltage suitable topower the electronic circuits in the OFID device 502. The energyreservoir 522, which can be a battery, a capacitor, or a super-capacitoris optional and would be necessary if the OFID device 502 is expected towork when an active reader is not present, there is insufficientelectrical current generated from the solar cell 506, or there is noneor insufficient incident light to power the OFID device 502. Dependingon the target application, an OFID device 502 may be outfitted with asensor interface, a processor, a timer or a serial communications port.

Referring to FIG. 6, another embodiment of an OFID communication system600 is shown. In this embodiment, a passive reader 602 is used insteadof the active reader 504 used in the previous embodiment. The passivereader 602 does not actively illuminate the solar cell 606 of the OFIDdevice 604. Instead, the OFID device 604 relies on incident light (shownas ambient light but it could be light generated from a reader) tostimulate PL emissions from the solar cell 606 or to charge the energyreservoir 608 of the OFID device 604. This embodiment has the advantageof simplifying the reader 602 complexity, which only needs to beequipped with a photo-detector 610 and the corresponding optics, such asan optical filter 612 and a lens.

The embodiment shown in FIG. 6 would be particularly suited for outdoorsettings with direct solar radiation. A drawback of this embodiment ofan OFID communication system 600 is that the passive reader 602 is notable to transmit data to the corresponding OFID devices 604. Hence, thepassive reader 602 cannot interrogate OFID devices 604 or synchronizetheir transmissions. In these cases, the OFID devices 604 may initiate atransmission based on events, such as a sensor input crossing athreshold or the expiration of a timer. It should also be known, that indark or not well-lit environments, the OFID device 604 of the presentembodiment can still transmit information to the passive reader 602 bystimulating EL emissions from the corresponding solar cell 606.

Referring to FIG. 7A, a schematic of system-level diagram of theinterface between a photo-luminescent (PL) modulator and energyharvester (EH) circuit 700, a solar cell 702 and a load (R_(load)). Thecircuit 700 can both modulate PL emissions from the solar cell 702 andharvest energy while boosting the voltage generated by said solar cell702. The device-to-reader input is a bit stream with data to betransmitted to a reader (not shown). FIG. 7B shows a possible PLmodulation circuit 704 diagram for the basic PL modulation principle ofthe circuit 700. The PL modulator and EH circuit 700 modulates PLemissions by varying the value of a resistance (R_(eq)) connected to thesolar cell 702. Varying R_(eq) varies V_(D) and consequentially Φ_(lum).Variations in R_(eq) are made according to the transmitted data. Fromthe PL modulation circuit 704 the following equation can be obtained:

$\begin{matrix}{{I_{ph} + I_{s}} = {{I_{s}e^{{V_{D}{({R_{sr} + R_{eq}})}}/{({{nV}_{T}R_{eq}})}}} + {\left( \frac{R_{sr} + R_{eq} + R_{sh}}{R_{eq}R_{sh}} \right)V_{D}}}} & (11)\end{matrix}$FIG. 7C is a graphical representation of the solution for equation (11),where I_(ph)=20 mA, along with the relationship between V_(D) andR_(eq). In FIG. 7C, the conditions V_(D)=V_(oc) and R_(eq)=R_(mpp),where

$R_{mpp} = \frac{V_{mpp}}{I_{mpp}}$is the impedance of the solar cell at the MPP and are highlighted. Togain this result, the following explicit expression for I_(mpp) wasemployed:

$\begin{matrix}{I_{mpp} = {I_{ph} + I_{s} - \frac{e\left( {I_{ph} + I_{s}} \right)}{w\left( \frac{e\left( {I_{ph} + I_{s}} \right)}{I_{s}} \right)}}} & (12)\end{matrix}$

From FIG. 7C it can be seen that there is a rapid increase in Φ_(lum)for R_(eq)>R_(mpp) with Φ_(lum) reaching 99.5% of its maximum OC valueat R_(eq)=10⁴Ω. Based on that result, OOK modulation can be accomplishedas follows: to transmit a logic high, set R_(eq)=R_(mpp) and to transmita logic low, set R_(eq)>10⁴Ω. Since R_(eq)=R_(mpp) results in themaximum amount of energy draw from the solar cell 702, when data is notbeing transmitted, R_(eq) should be set to R_(mpp). PAM can beaccomplished by varying R_(eq) in discrete steps in the range betweenR_(mpp) and 10⁴Ω.

Referring to FIG. 8, a schematic diagram of an implementation of a PLmodulator and EH circuit 800 is shown. The PL modulator and EH circuit800 is based on one of the power conversion schemes discussed above; inthis case a boost DC-DC converter 802. Transistor (e.g., Metal OxideField Effect Transistor (MOSFET)) M₁ works as a switch and is driven byclock signal ϕ of frequency f_(sw) and duty cycle ρ. The circuit 800works in two phases: Phase 1 when ϕ is logic high and Phase 2 when ϕ islogic low. During Phase 1, M₁ closes connecting inductor L in parallelwith the solar cell 804, charging inductor L. In Phase 2, M₁ opens andthe inductor discharges through the Schottky diode D_(s) into the loadR_(load). During Phase 1 capacitor C_(L) feeds the load. The pulse widthmodulator 806 generates a clock signal with duty cycle proportional atits input V_(mod). The data being transmitted is encoded by the digitalsignal d_(in), which in this figure, is shown controlling V_(mod) and,therefore, the duty cycle of the boost DC-DC converter 802. The inputimpedance R_(in) of the DC-DC converter 802 is a function of the dutycycle ρ. The duty cycle plays an important role in the operation of theDC-DC converter 802. The proper value of the duty cycle enables themaximum energy transfer from the solar cell 804 to the load.

In order to analyze the proposed PL modulator and EH circuit 800 shownin FIG. 8, equivalent linearized circuits at various phases andconditions were analyzed and are shown in FIGS. 9A-9D. FIG. 9A is alinearized solar cell equivalent circuit 900 where the diode in thesolar cell equivalent circuit 102 of FIG. 1 was linearized using thefirst two Taylor series terms of its current-vs-voltage curve. Variablesg₁ and g₂ are equal to:

$\begin{matrix}{g_{1} = {\frac{I_{s}}{nVT}e^{V_{dm}/{nV}_{T}}}} & (12) \\{g_{2} = {{I_{s}\left( {e^{\frac{V_{dm}}{{nV}_{T}}} - 1} \right)} - {\frac{V_{dm}I_{s}}{{nV}_{T}}e^{\frac{V_{dm}}{{nV}_{T}}}}}} & (13)\end{matrix}$and V_(dm) is the solar cell's DC operating point. By replacing thesolar cell 804 of the proposed PL modulator and EH circuit 800 of FIG. 8with the linearized equivalent 900 for Phase 1 (ϕ=high) yields theequivalent circuit 910 shown in FIG. 9B. In this figure,R_(p)=R_(sh)//(1/g₁), R_(si) is the series resistance of the inductorand R_(on) is the on resistance of M₁. By analyzing the equivalentcircuit 910 the following set of differential equations are obtained:

$\begin{matrix}{\frac{{dV}_{d}}{dt} = {{{- \frac{1}{C_{j}}}\left( {\frac{1}{R_{P}} + \frac{1}{R_{sr}}} \right)V_{d}} + {\frac{1}{C_{j}R_{sr}}V_{c}} + \frac{I_{ph} - g_{2}}{C_{j}}}} & (14) \\{\frac{{dV}_{c}}{dt} = {{\frac{1}{C_{P}R_{sr}}V_{d}} - {\frac{1}{C_{P}R_{sr}}V_{c}} - {\frac{1}{C_{P}}I_{i}}}} & (15) \\{\frac{{dI}_{i}}{dt} = {{{- \frac{1}{L}}V_{c}} - {\frac{R_{si} + R_{on}}{L}I_{i}}}} & (16) \\{\frac{{dV}_{out}}{dt} = {{- \frac{1}{R_{load}C_{L}}}V_{out}}} & (17)\end{matrix}$

For Phase 2, it is important to consider two different modes: ContinuousCurrent Mode (CCM) and Discontinuous Current Mode (DCM), In CCM, theinductor L doesn't become fully discharged during Phase 2, i.e. thecurrent through the inductor, I_(i), remains greater than zero. FIG. 9Cshows the equivalent circuit 920 during Phase 2 for CCM. V_(don) modelsthe voltage drop across the Schottky diode. By analyzing the equivalentcircuit 920 the following set of differential equations are obtained:

$\begin{matrix}{\frac{{dV}_{d}}{dt} = {{{- \frac{1}{C_{j}}}\left( {\frac{1}{R_{P}} + \frac{1}{R_{sr}}} \right)V_{d}} + {\frac{1}{C_{j}R_{sr}}V_{c}} + \frac{I_{ph} - g_{2}}{C_{j}}}} & (18) \\{\frac{{dV}_{c}}{dt} = {{\frac{1}{C_{j}R_{sr}}V_{d}} - {\frac{1}{C_{p}R_{sr}}V_{c}} - {\frac{1}{C_{P}}I_{i}}}} & (19) \\{\frac{{dI}_{i}}{dt} = {{{- \frac{1}{L}}V_{c}} - {\frac{1}{L}V_{out}} - \frac{V_{don}}{L} - {R_{si}I_{i}}}} & (20) \\{\frac{{dV}_{out}}{dt} = {{\frac{1}{C_{L}}I_{i}} - {\frac{1}{R_{load}C_{L}}V_{out}}}} & (21)\end{matrix}$FIG. 9D shows the equivalent circuit 930 during Phase 2 for DCM. In DCM,the inductor gets fully discharged at some point during Phase 2. Whenthat happens, the diode D_(s) stops conducting, causing the load tobecome isolated from the rest of the circuit and prevent C_(L) fromdischarging through the inductor. By analyzing the equivalent circuit930 the following set of differential equations are obtained:

$\begin{matrix}{\frac{{dV}_{d}}{dt} = {{{- \frac{1}{C_{j}}}\left( {\frac{1}{R_{P}} + \frac{1}{R_{sr}}} \right)V_{d}} + {\frac{1}{C_{j}R_{sr}}V_{c}} + \frac{I_{ph} - g_{2}}{C_{j}}}} & (22) \\{\frac{{dV}_{c}}{dt} = {{\frac{1}{C_{P}R_{sr}}V_{d}} - {\frac{1}{C_{P}R_{sr}}V_{c}}}} & (23) \\{\frac{{dV}_{out}}{dt} = {{- \frac{1}{R_{load}C_{L}}}V_{out}}} & (24)\end{matrix}$

During analysis, equations (14) to (24) were solved numerically usingthe Runge-Kutta method implemented by computer software, specificallyMATLAB®, however other software and methods known to one having ordinaryskill in the art may also be used. The equations were solved followingthe iterative process shown in FIG. 9E. In FIG. 9E:

“i.c.” stand for initial conditions,

“f.c.” stands for final conditions,T _(sw)=1/f _(sw),T ₁ =ρT _(sw),T ₂=(1−ρ)T _(sw), andT_(sim) is the simulation time. Using the iterative process shown inFIG. 9E, the equations (14-24) describing the DC-DC converter 802 (shownin FIG. 8) were solved for three different values of I_(ph) (10, 15 and20 mA) and for L=68 μH, R_(si)=1.0Ω, C_(P)=10 μF, R_(on)=0.3Ω,f_(clk)=40 kHz, V_(don)=0.3 V, R_(sr)=1Ω, C_(j)=550 nF, R_(load)=5 kΩ,and T_(sim)=200 ms. FIG. 9F is a graph which shows the resulting averageoutput voltage V_(out) as a function of the duty cycle ρ. It can beobserved that the duty cycle at which the output voltage peaks, i.e.maximum power is transferred to the load, varies with the photo-currentI_(ph). Setting the duty cycle such that maximum power is transferred tothe load is the function of an MPP controller.

Referring to FIG. 10A, the relationship between the input impedance ofthe DC-DC converter 802 (shown in FIG. 8), R_(in), and the duty cycle ofthe clock signal ρ is presented. This relationship was obtained by firstapproximating the input impedance of the DC-DC converter 802, R_(in) inthe following manner:

$\begin{matrix}{R_{in} = \frac{\overset{\_}{V_{c}}}{\left( \frac{Q_{i}}{T_{sw}} \right)}} & (25)\end{matrix}$where V _(c), is the average V_(c) voltage, andQ_(i) is the charge drawn by the inductor in one clock cycle. The nextstep in obtaining the relationship was done by assuming a linear profilefor the inductor current during Phase 1, when the inductor is charging,and letting I_(i1) and I_(i2) be the inductor currents at the beginningof phases 1 and 2, respectively, the following equation is obtained:

$\begin{matrix}{Q_{i} = {{\int_{0}^{T_{1}}{\left( {I_{i\; 1} + \frac{I_{i\; 2} - I_{i\; 1}}{T_{1}}} \right){dt}}} = {{I_{i\; 1}T_{1}} + {\frac{T_{1}}{2}\left( {I_{i\; 2} - I_{i\; 1}} \right)}}}} & (26)\end{matrix}$The final step in obtaining the relationship seen in FIG. 10A was tosubstitute Q_(i) from (26) into (25) and use the values of V _(c),I_(i2), and I_(i1) obtained from solving the equation describing theDC-DC converter. In FIG. 10A, the impedance of the solar cell 804 (shownin FIG. 8) at the MPP, R_(mpp), is marked for I_(ph)=10, 15, and 20 mA.It should be observed that an input impedance greater than 10⁴Ω isachieved for ρ≤2.5%. Furthermore, by combining the relationship forΦ_(lum) vs. R_(in) from FIG. 7C and the relationship for R_(in) vs. ρ inFIG. 10A yields the relationship for Φ_(lum) vs. ρ shown in FIG. 10B.FIG. 10B shows that the PL radiant flux of a solar cell 804 can bemodulated by varying the duty cycle of the DC-DC converter 802 (shown inFIG. 8).

It is also possible to improve the efficiency of the boost DC-DCconverter of FIG. 8 by replacing the Schottky diode with an active diodecircuit in order to reduce the losses due to the voltage drop across theSchottky diode. An embodiment of an active diode circuit includes aMOSFET and a voltage comparator. The comparator turns off the MOSFETwhen its drain-to-source voltage becomes negative. Although thecomparator consumes additional power, an on-chip low-powerimplementation of a DC-DC converter with an active diode is moreefficient than an implementation with discrete components. Observationswere also made, through testing, that gallium nitride (GaN) blue andgreen LEDs have a strong PL response when illuminated with violet light(405 nm). This PL response is also a function of the impedance acrossthe LED. Hence, in one embodiment, a proposed PL modulator and EHcircuit can be used to modulate the PL emissions of GaN blue and greenLEDs. Typically, LEDs have much smaller active areas and higher cost(per unit area) than solar cells. Thus, it may be more beneficial to useLEDs for OFID devices where highly directed and concentrated light beamscan be used.

Further testing was performed to verify the functionality and validityof analytical models previously presented for an embodiment of the PLmodulator and EH circuit. FIG. 11A shows a schematic diagram of the testsetup 1100 used to conduct such tests. A GaAs solar cell 1102 was placedin front of a flashlight 1104 having a red (623 nm) LED 1106. The lens1108 of the LED 1106 was at a distance d from the solar cell, where inone embodiment the distance d was 12 centimeters. In order to operatethe flashlight 1104 it was connected to a power supply unit 1116. Asecond, modified, flashlight 1110, configured to hold a photo-diode 1112and an amplifier, was also placed in front of the GaAs solar cell 1102.An 850 nm longpass filter 1114 was attached to the modified flashlight1110 at the end facing the GaAs solar cell 1102. The test setup 1100further included a PL modulator and EH circuit 1118 which was connectedto an oscilloscope 1122 and waveform generator 1124. The PL modulatorand EH circuit 1118 was capable of generating a clock signal provided bya voltage-controlled pulse width modulator circuit 1128. A receiver 1120was also included in the test setup 1100 which was connected to theoscilloscope 1122 and comprising a trans-impedance amplifier, twoinverting amplification stages and a low-pass filter 1126. The low-passfilter 1126, was used to remove ripple noise caused by the switchingaction of the DC-DC converter.

Current through the LED 1106 was adjusted such that the photo-generatedcurrent in the solar cell I_(ph) was 20 mA. The photo-diode's 1112current I_(pd) was converted to a voltage by a trans-impedance amplifierand further amplified to produce the voltage V_(pl), which isproportional to I_(pd). The oscilloscope 1122 was used to record theV_(pl), V_(mod), and V_(out) waveforms. The photo-diode's 1112 currentI_(pd) is a function of the solar cell's 1102 radiant flux Φ_(lum) atdistance d, Φ_(lum) (d), as follows:I _(pd) =R _(pd)Φ_(lum)(d)  (27)where, R_(pd) is the responsivity of the photo-diode 1112 in A/W. Giventhat Φ_(lum) (d) ∝Φ_(lum), it can be concluded that V_(pl)∝Φ_(lum).Hence, the V_(pl) waveforms recorded by the oscilloscope wereproportional to the luminescent radiant flux emitted by the solar cell1102. Returning to the pulse width modulator circuit 1128 of the PLmodulator and EH circuit 1118, the duty cycle of the clock signalgenerated is a linear function of the voltage V_(mod) with V_(mod)=95 mVcorresponding to ρ=0% duty cycle and V_(mod)=900 mV corresponding toρ=100% duty cycle. The switching frequency was set to 40 kHz. Thelow-pass filter 1126 implemented was an eighth order Butterworthlow-pass filter with a cutoff frequency set to 20 kHz. Other parametersof the receiver 1120 were set as follows: R₁=100 kΩ, C₁=11 pF, R₂=10 kΩ,C₂=62 pF, R₃=1 MΩ, R₄=9.8 kΩ, R₅=9.8 kΩ, and V_(ref)=1.5 V.

The first test conducted involved sweeping V_(mod) from 95 mV to 900 mVto observe the effects of duty cycle change on V_(pl) and on the outputof the DC-DC converter, V_(out). FIG. 11B shows the recorded V_(mod),V_(pl), and V_(out) waveforms. The inset 1140 shows a closeup view ofthe ripple noise in V_(pl), which is due to the switching action of theDC-DC converter. The dotted line is the low-pass version of V_(pl) withthe ripple removed. It should be observed that, V_(pl), which isproportional to Φ_(lum), achieves its maximum for ρ=0% or V_(mod)=95 mV(OC) and its minimum for ρ=100% or V_(mod)=900 mV (SC). The MPP, whichis the point at which V_(out) is maximum is achieved for V_(mod)=355 mVor ρ=32.7% duty cycle. The location of the MPP and the overall behaviorof V_(out) witch respect to ρ is in good agreement with the resultspresented in FIG. 9F. Furthermore, the V_(pl) waveform, after ripplenoise was removed, is also in good agreement with the relationshipbetween Φ_(lum) and the duty cycle p presented in FIG. 10B. At the MPP,V_(out)=8 V, resulting in 11.9 mW of power delivered to the load.Considering that for I_(ph)=20 mA, V_(pp)×I_(mpp)=17.7 mW, theefficiency of the DC-DC converter, at the MPP, is 67%.

A second test that included varying the amplitude of the emitted PLradiant flux in discrete steps was also carried out. This test shows thepossibility of using PAM to transmit digital information with PLemissions of a solar cell 1102. To this end, V_(mod) was varied in eightdiscrete levels, wherein each level corresponded to a digital symbolranging from 95 mV (OC) to 355 mV (MPP). Due to the non-linearrelationship between Φ_(lum) the duty cycle ρ, the levels were notequally spaced. Table 1 lists the V_(mod) voltage values and thecorresponding duty cycles assigned to each symbol.

TABLE 1 V_(mod) and ρ values for transmitted symbols Symbol V_(mod) (mV)ρ (%) 000 95 0 001 165 8.7 010 210 14.7 011 240 18.7 100 260 21.3 101285 24.4 110 315 28.1 111 355 32.7With further reference to the second test performed, FIG. 11C shows theV_(mod), V_(pl), and V_(out) waveforms recorded by the oscilloscope forthat test. The inset here 1160 shows a closeup view of the ripple noisein V_(pl). At the receiver side, this noise is removed by the low-passfilter 1126 for reliable reception of the transmitted symbols. FIG. 11Dshows graphically the output of the low-pass filter 1126 {tilde over(V)}₁p, and, as seen in the figure, a cleaner more distinguishablesignal with the eight discrete levels can be identified. In this secondtest, the duration of each symbol was set to 0.625 ms, which wouldresult in a transmissions speed of 4.8 kbps. FIG. 11C also shows thatthe output of the DC-DC converter dropped to 5.99 V, as a result of themodulation, resulting in 6.54 mW of power delivered to the load. Thisdrop is due to the fact that, in order to modulate the PL emissions ofthe solar cell 1102 by varying the input impedance of the DC-DCconverter, the DC-DC converter does not always operate at the MPP. Thisresult illustrated an inherent trade-off in the proposed PL modulatorand EH circuit 1118, namely, the ability of harvesting maximum power istraded with the ability of transmitting information.

Depending on the requirements of the target application, either powerharvesting or information transmission may be maximized. For example, tomaximize power harvesting while transmitting information, the variationsin duty cycle should be kept close to the MPP. This would mean thateither: 1) fewer discrete levels (symbols) are transmitter per unit timeor 2) the spacing between each level is reduced. In either case thetransmission of information is hindered by a reduction of thetransmission rate or a reduction of the signal-to-noise ratio.

A third test was carried out to demonstrate the performance of OOKmodulation. In this test the PL radiant flux was modulated by flippingV_(mod) between 95 mV (OC) and 355 mV (MPP). FIG. 11E shows the recordedV_(pl) waveforms for three different modulation frequencies: 1 kHz (toprow), 5 kHz (middle row), and 10 kHz (bottom row). In each row, V_(pl)and {tilde over (V)}_(pl) in volts are plotted vs. time in ms. It can beseen from this figure, that as the modulation frequency increases, theamplitude of V_(pl) decreases. This decrease in amplitude is due tocapacitor 1130 which limits how fast the DC-DC converter can reach itssteady state after a change in the duty cycle. This is another exampleof a trade-off in the PL modulator and EH circuit 1118, depending on therequirements of the target applications, the value of capacitor 1130should be set to either maximize harvested power or maximize thetransmission bandwidth.

Referring to FIG. 12, a schematic diagram of a PL modulator circuit1200, according to one embodiment of the present disclosure, is shown.The PL modulator circuit 1200 comprises: a power converter 1202, lineencoder 1204 and an AND logic gate 1206. The PL modulator circuit 1200further comprises a load/charger circuit 1208, an energy reservoir 1210,an MPP controller 1212, a pulse modulator 1214, and a solar cell 1216.The power converter 1202 includes an input, a clock input and an output.The output of the power converter 1202 feeds into the input of the MPPcontroller 1212 and the input of load/charger circuit 1208. Theload/charger circuit 1208 also has an output that is connected to theenergy reservoir 1210 and is adapted to transfer charge to the energyreservoir 1210. The output of the MPP controller 1212 feeds into theinput of the pulse modulator 1214, and the output of the pulse modulator1214 feeds into one terminal of the AND logic gate 1206. Data feeds intothe line encoder 1204, which then encodes said data and outputs theencoded data to the remaining terminal of the AND logic gate 1206. Inthis embodiment, the MPP controller 1212 continuously adjusts the dutycycle or the frequency of a sequence of pulses generated by the pulsemodulator 1214. The output of the AND logic gate 1206 drives the clockinput of the power converter 1202. The solar cell circuits of the priorart do not include the ability to modulate luminescent emissions and assuch do not include the line encoder 1204 or the AND gate 1206, as shownin FIG. 28.

Referring to FIG. 13, a schematic diagram of a PL modulator circuit1300, according to one embodiment of the present disclosure, is shown.The PL modulator circuit 1300 comprises: a power converter 1302, abipolar transistor 1304 within a feedback loop, and a switch S1. The PLmodulator circuit 1300, further comprises a symbol-to-voltage mappingcircuit 1306, a pulse modulator 1308, a load/charger circuit 1310, anenergy reservoir 1312, an MPP controller 1314 and a solar cell 1316. Thepower converter 1302 has an input, a clock input, and an output. Theoutput of the power converter 1302 feeds into an input of the MPPcontroller 1314 and the input of the load/charger circuit 1310. Theoutput of the load/charger circuit 1310 feeds into the energy reservoir1312, making it possible for the load/charger circuit 1310 to storeexcess energy in the energy reservoir 1312. The MPP controller 1314output connects to the input of the pulse modulator 1308 and the outputof the pulse modulator 1308 drives the clock input of the powerconverter 1302. The PL modulator circuit 1300 operates in two modes:transmission mode (TX) and energy harvesting mode (EH). In EH mode, theswitch S1 is closed, the bipolar transistor 1304 is turned off and theMPP controller 1314 is enabled. In TX mode, the switch S1 is open, theMPP controller 1314 is disabled and the base of the bipolar transistor1304 is driven by the feedback loop such that the voltage across thesolar cell 1316 varies according to the data being transmitted.

Referring to FIG. 14, a schematic diagram of a PL modulator circuit1400, according to one embodiment of the present disclosure, is shown.FIG. 14 is similar to FIG. 13 with some difference, thereforeduplicative description of the connectivity is avoided. The PL modulatorcircuit 1400 comprises: a power converter 1402, a MOSFET 1404 within afeedback loop, and a switch S1. The PL modulator circuit 1400 furthercomprises: a symbol-to-voltage mapping circuit 1406, a pulse modulator1408, a load/charger circuit 1410, an energy reservoir 1412, an MPPcontroller 1414, and a solar cell 1416. The PL modulator circuit 1400operates in two modes: a transmission mode (TX) and an energy harvestingmode (EH). In EH mode, the switch S1 is closed, the MOSFET 1404 isturned off and the MPP controller 1414 is enabled. In TX mode, theswitch S1 is open, the MPP controller 1414 is disabled and the gate ofthe MOSFET 1404 is driven by the feedback loop such that the voltageacross the solar cell 1416 varies according to the data beingtransmitted.

Referring to FIG. 15, a schematic diagram of a PL modulator circuit1500, according to one embodiment of the present disclosure, is shown.The PL modulator circuit 1500 comprises: a power converter 1502, avariable oscillator 1504, an MPP controller 1506 and asymbol-to-frequency mapping circuit 1508 that assigns a frequency toeach symbol in a set of data to be transmitted. The PL modulator circuit1500 further comprises: a load/charger circuit 1510, an energy reservoir1512 and a solar cell 1514. The frequency output of the variableoscillator 1504 is a function of the data being transmitted and theoutput of the MPP controller 1506. The symbol-to-frequency mappingcircuit 1508 assigns a frequency to each symbol in the set of data beingtransmitted such that input impedance of the power converter 1502changes from short circuit to a very large impedance (i.e. greater than10 kOhms).

Referring to FIG. 16, a schematic diagram of a PL modulator circuit1600, according to one embodiment of the present disclosure, is shown.The PL modulator circuit 1600 comprises: a power converter 1602, a pulsewidth modulator 1604, an MPP controller 1606, and a symbol-to-duty-cyclemapping circuit 1608 that assigns a duty cycle or pulse width to eachsymbol in a set of data to be transmitted. The PL modulator circuit 1600further comprises: a load/charger circuit 1610, an energy reservoir1612, and a solar cell 1614. The duty cycle of the pulse width modulator1604 is a function of the data being transmitted and the output of theMPP controller 1606. The symbol-to-duty-cycle mapping circuit 1608assigns a duty cycle to each symbol in the set of data being transmittedsuch that input impedance of the power converter 1602 changes from shortcircuit to a very large impedance (i.e. an impedance greater than 10kOhms).

Referring to FIG. 17, a schematic diagram of a PL modulator circuit1700, according to one embodiment of the present disclosure, is shown.The PL modulator circuit 1700 comprises: a power converter 1702, adigital potentiometer 1704, an analog-to-digital (A/D) converter 1706, asymbol-to-voltage mapping circuit 1708 and a switch S1. The PL modulatorcircuit 1700 further comprises: a pulse modulator 1710, a load/chargercircuit 1712, an energy reservoir 1714, an MPP controller 1716, and asolar cell 1718. The PL modulator circuit 1700 operates in two modes:transmission mode (TX) and energy harvesting mode (EH). In EH mode theswitch S1 is closed, the MPP controller 1716 is enabled and the outputfrom the digital potentiometer 1704 is set to high impedance. In TX modethe switch S1 is open, the MPP controller 1716 is disabled and theoutput resistance from the digital potentiometer 1704 is set such thatthe voltage across the solar cell 1718 closely follows the voltageoutput of the symbol-to-voltage mapping circuit 1708. Thesymbol-to-voltage mapping circuit 1708 assigns a voltage to each symbolin a data stream which is to be transmitted such that the voltage outputof the symbol-to-voltage mapping circuit 1708 spans the range from 0 Vto the open circuit (OC) voltage of the solar cell 1718.

Referring to FIG. 18, a schematic diagram of a PL modulator circuit1800, according to one embodiment of the present disclosure, is shown.The PL modulator circuit 1800 comprises: a power converter 1802, twoswitches S1 and S2, a line encoder 1804, a switch driver 1806, aload/charger circuit 1808, an energy reservoir 1810, an MPP controller1812, a pulse modulator 1814, and a solar cell 1816. The PL modulatorcircuit 1800 operates in two modes: transmission mode (TX) and energyharvesting mode (EH). In EH mode S1 is closed, the MPP controller 1812is enabled and S2 is open. In TX mode, the following two sub-modes arepossible: S1 remains open and S2 opens and closes according to the databeing transmitted, S1 and S2 open and close out of phase (i.e. when S1is open, S2 is closed) according to the data being transmitted. In thesub-mode when S1 is open and S2 is closed there is a short circuit (SC)event leading to a maximum current draw resulting in an almost 0 Vacross the solar cell. This results in a minimum amount of luminescencebeing generated by the solar cell 1816. Such luminescence is infraredfor materials such as GaAs or CdTe. However, perovskite solar cell mayprovide luminescence in the visible range, i.e. red or green. In theevent that both switches S1 and S2 are open, resulting in an opencircuit (OC) event the impedance across the solar cell 1816 is infiniteand the solar cell 1816 achieves a maximum luminescent emission. In thescenario that switch S1 is closed and switch S2 is open, the powerconverter 1802 applies a level of impedance equivalent to the MPP to thesolar cell 1816, resulting in a median level of luminescent emissionsranging between minimum and maximum emissions. The MPP controller 1812may be disabled when S1 is open.

Referring to FIG. 19, a schematic diagram of a PL modulator circuit1900, according to one embodiment of the present disclosure, is shown.The PL modulator circuit 1900 comprises: a power converter 1902, avoltage buffer 1904, a symbol-to-voltage mapping circuit 1906, a switchS1, a load/charger circuit 1908, an energy reservoir 1910, an MPPcontroller 1912, a pulse modulator 1914, and a solar cell 1916. Thecircuit operates in two modes: transmission mode (TX) and energyharvesting mode (EH). In EH mode the switch S1 is closed, the MPPcontroller 1912 is enabled and the output from the voltage buffer 1904is set to high impedance. In TX mode the switch S1 is open, the MPPcontroller 1912 is disabled and the output of the voltage buffer 1904follows the output of the symbol-to-voltage mapping circuit 1906. Thesymbol-to-voltage mapping circuit 1906 assigns a voltage to each symbolin a data stream being transmitted such that its voltage output spansthe range from 0 V to the open circuit (OC) voltage of the solar cell1916.

According to the present disclosure, there are numerous potentialapplications for an OFID communications system. One potentialapplication for an OFID communication system is environmentalmonitoring. The OFID communication system could be equipped with sensorsto monitor environmental variables such as water or air contaminants.According to this application, solar energy could be used for bothpowering the sensor and for stimulating PL emissions from the solarcells that can be modulated with data from the sensors. Furthermore, itwould be possible for an unmanned air vehicle (UAV) equipped with apassive imaging receiver, such as a high-speed infrared camera, todetect the modulated PL emissions of the solar cells. The camera wouldbe able to receive and spatially separate several transmissionssimultaneously and be able to locate the position of the sensors todetermine their location relative to the surroundings. Multiple sensorunits could be deployed over an area to provide greater range. OFIDcommunications systems also offer a potential use in identifying andtracking large objects such as shipping containers as they move througha port. In this application, an active reader with a high-powercollimated light beam, possibly from a laser source, could be used tointerrogate an OFID tag, attached to the shipping container, from a longdistance. In this application the OFID communication system would notrequire active power from batteries, supporting its long-term andsustainable usage on objects used for transportation of goods over along distance such as, shipping, trucking or railway containers.

Another possible application of an OFID communication system is thetracking and monitoring of perishable goods, such as foods or vaccines,throughout the different points of a supply chain. According to thisapplication, as a package carrying said perishable good moves throughthe supply chain, its presence at different points in the supply chaincan be detected by optically interrogating an OFID communication system.Moreover, the OFID communication system can be equipped with at leastone temperature sensor to determine if the goods have been kept underrecommended conditions. The on-board solar cell would allow the OFIDcommunication system to stay active and record temperature, even when itis not within range of a reader, by harvesting incident light energy.OFID communication systems also offer potential applications insmart-home environments. In these environments OFID communicationsystems could monitor variables such as temperature, noise, lightintensity, air quality or human presence while their on-board solarcells harvest energy from incident light. In this application, PL or ELemissions from the solar cell could be modulated according to the sensedvariables. A passive reader equipped with an imaging receiver can beemployed to receive multiple luminescence emissions and pinpoint theirlocations within the room.

Referring to FIGS. 20 and 21, two embodiments of the PL modulationincluding tandem solar cells according to the present disclosure areprovided. In these two embodiments, solar cells are provided in a tandemmanner. Tandem solar cells are known. Such arrangements are providedwhen one solar cell, e.g., the top transparent or semi-transparent solarcell, is particularly adapted to absorb light at a first spectral range,while the second solar cell, i.e., the bottom solar cell, is adapted toabsorb light that is transmitted through the first solar cell at asecond spectral range. Such arrangements provide more suitableefficiency across a wider range of light spectrum.

Referring to FIG. 20, the top solar cell is also adapted to PL modulatedata. That is, while the top solar cell is adapted to absorb light andprovide charge to the DC-DC converter, it is also adapted to emitluminescent radiation as discussed according to any of the datatransmission techniques discussed above. At the same time, the bottomsolar cell is adapted to provide charge to the DC-DC converter.

Referring to FIG. 21, the top solar cell and the bottom solar cell arealso adapted to PL modulate data. That is, the while the top solar cellis adapted to absorb light and provide charge to the DC-DC converter, itis also adapted to emit luminescent radiation as discussed according toany of the data transmission techniques discussed above. At the sametime, the bottom solar cell is adapted to provide charge to the DC-DCconverter, in addition it is adapted to emit luminescent radiation againas discussed according to any of the data transmission techniquesdiscussed above.

While the discussion above has been mainly directed to PL modulation, ELmodulation as discussed previously is also within the scope of thepresent disclosure. Towards this end, FIGS. 22-27 are presented toshowcase various embodiments of EL modulation; however, otherembodiments previously discussed in the priority document of the presentdisclosure (i.e., U.S. Provisional Patent Application Ser. No.62/727,315 filed Sep. 5, 2018), which is incorporated by reference inits entirety, is also within the scope of the present disclosure.

Referring to FIG. 22, an embodiment of the EL modulation circuit isprovided. The EL modulator shown in FIG. 22 uses a bidirectional powerconverter and can modulate both PL and EL. The TX/EH signal sets theoperation mode of the circuit. The operation mode can be set to datatransmission (TX) or to energy harvesting (EH). The TX/EH signal setsthe direction of power converter. In the forward direction the powerconverter transfers energy from the solar cell to the load or to acharger (EH mode). In the reverse direction the power convertertransfers energy from the energy reservoir toward the solar cell (TXmode). The duty cycle of the clock input of the power converter isvaried according to the data being transmitted.

Referring to FIG. 23, another embodiment of the EL modulator ispresented. This EL modulator uses a bidirectional power converter withvariable reference voltage input (ref). The TX/EH signal sets theoperation mode of the circuit. The operation mode can be set to datatransmission (TX) or to energy harvesting (EH). The TX/EH signal setsthe direction of power converter. In the forward direction the powerconverter transfers energy from the solar cell to the load or to acharger (EH mode). In the reverse direction the power convertertransfers energy from the energy reservoir toward the solar cell (TXmode). The reference input voltage (ref) of the power converter is setsuch that the current through the solar cell varies according to thedata being transmitted.

Referring to FIG. 24, another embodiment of the EL modulator ispresented. This EL modulator uses a bidirectional power converter and aswitch (S1) between the solar cell and the power converter. The TX/EHsignal sets the operation mode of the circuit. The operation mode can beset to data transmission (TX) or to energy harvesting (EH). The TX/EHsignal sets the direction of power converter. In the forward directionthe power converter transfers energy from the solar cell to the load orto a charger (EH mode). In the reverse direction the power convertertransfers energy from the energy reservoir toward the solar cell (TXmode). Switch S1 is controlled by the output of the line encoder. Thereference input voltage of the power converter is set to a fixed value.

Referring to FIG. 25, another embodiment of the EL modulator ispresented. This EL modulator uses a power converter, a programmablecurrent source and two switches (S1 and S2). The TX/EH signal sets theoperation mode of the circuit. The operation mode can be set to datatransmission (TX) or to energy harvesting (EH). In EH mode S1 is closed,S2 is open and the MPP controller is enabled. In TX mode S1 is open, S2is closed and the MPP controller is disabled. In TX mode the currentgenerated by the programmable current source is a function of the databeing transmitted.

Referring to FIG. 26, another embodiment of the EL modulator ispresented. This EL modulator uses a power converter, a voltage regulatorand digital-to-analog converter (D/A). The TX/EH signal sets theoperation mode of the circuit. The operation mode can be set to datatransmission (TX) or to energy harvesting (EH). In EH mode S1 is closed,S2 is open and the MPP controller is enabled. In TX mode S1 is open, S2is closed and the MPP controller is disabled. The reference inputvoltage (ref) of the voltage regulator is set such that the currentthrough the solar cell varies according to the data being transmitted.

Referring to FIG. 27, another embodiment of the EL modulator ispresented. This EL modulator uses a power converter, a voltage regulatorand two switches (S1 and S2). The circuit operates in two modes:transmission mode (TX) and energy harvesting mode (EH). In EH mode S1 isclosed, S2 is open and the MPP controller is enabled. In TX mode thereare two sub-modes: No ambient light: in this sub-mode S1 remains openand S2 opens and closes according to the data being transmitted. Ambientlight present: in this sub-mode S1 and S2 open and close out of phaseaccording to the data being transmitted. The MPP controller is enabledwhenever S1 is closed. The reference input voltage (VREF) of the voltageregulator is set to a constant value.

Those having ordinary skill in the art will recognize that numerousmodifications can be made to the specific implementations describedabove. The implementations should not be limited to the particularlimitations described. Other implementations may be possible.

The invention claimed is:
 1. An energy-harvesting system capable ofwireless communication, comprising: a solar cell adapted to generatecharge from light incident on the solar cell; an energy-harvestingcircuit coupled to the solar cell, the energy-harvesting circuit adaptedto convey charge from the solar cell to an energy reservoir; a datacommunication circuit adapted to encode data to be communicated by thesolar cell by adjusting voltage across the solar cell into at least twostates including 1) an open circuit (OC) state, where the currentthrough the solar cell is substantially zero; and 2) a maximum powerpoint (MPP) state, where power (V×I) drawn from the solar cell ismaximum, wherein “substantially zero” is defined as less than 5% ofcurrent of the MPP state, wherein the solar cell in response to thechanges in its voltage and the incident light emits luminescentradiation with an intensity corresponding to the at least two states,thereby forming a wireless transmitter.
 2. The energy-harvesting systemof claim 1, where the data communication is based on digital binarycommunication.
 3. The energy-harvesting system of claim 1, wherein theat least two states further includes: 3) a short circuit (SC) state,where the voltage across the solar cell is substantially zero, and thedata communication is based on digital ternary communication.
 4. Theenergy-harvesting system of claim 1, wherein the at least two statesfurther includes a plurality of other states between the short circuitstate and an open circuit state, where the current through the solarcell is substantially zero, and the communication is based on a digitalM-ary modulation scheme.
 5. The energy-harvesting system of claim 1,wherein the voltage across the solar cell is continuously adjustedbetween the short circuit state, where the voltage across the solar cellis substantially zero, and the open circuit state, where the currentthrough the solar cell is substantially zero, and the communication isbased on an analog modulation scheme.
 6. The energy-harvesting system ofclaim 1, wherein the energy-harvesting system is based on a powerconverter selected from the group consisting of a boost DC-DC converter,a buck-boost DC-DC converter, a buck DC-DC converter, and a combinationthereof.
 7. The energy-harvesting system of claim 3, wherein power ofthe emitted luminescence vs. voltage across the solar cell is about 0.12μW at about 0 V for the SC state, about 0.25 μW at about 0.92 volt forthe MPP state, and about 4.8 μW at about 1.02 V for the OC state,wherein “about” is defined as a degree of variability in a value orrange within ±10% of an associated value.
 8. The energy-harvestingsystem of claim 1, further comprising a receiver configured to receivethe solar cell-transmitted data from the solar cell.
 9. Theenergy-harvesting system of claim 8, wherein the receiver is furtherconfigured to wirelessly communicate data in form of incident lightmodulation to the solar cell and the solar cell further configured toreceive the incident light and decode data associated with thereceiver-transmitted data.
 10. The energy-harvesting system of claim 9,wherein the energy-harvesting circuit is configured to harvest energyfrom the solar during wireless communication from the receiver.
 11. Amethod of harvesting energy while providing wireless communication,comprising: receiving incident light by a solar cell adapted to generatecharge from the incident light incident on the solar cell; harvestingenergy from the solar cell by an energy-harvesting circuit coupled tothe solar cell; conveying charge from the solar cell to an energyreservoir by the energy-harvesting circuit; adjusting electricalimpedance across the solar cell into at least two impedance statesincluding 1) an open circuit (OC) state, where the current through thesolar cell is substantially zero; and 2) a maximum power point (MPP)state, where power (V×I) drawn from the solar cell is maximum, wherein“substantially zero” is defined as less than 5% of current of the MPPstate, wherein the solar cell in response to the presented impedance andthe incident light emits luminescent radiation with an intensitycorresponding to the at least two impedance states; and wirelesslycommunicating data based on the at least two impedance states.
 12. Themethod of claim 11, where the data communication is based on digitalbinary communication.
 13. The method of claim 11, wherein the at leasttwo states further includes: 3) a short circuit (SC) state, where thevoltage across the solar cell is substantially zero, and the datacommunication is based on digital ternary communication.
 14. The methodof claim 11, wherein the at least two states further includes aplurality of other states between the short circuit state and an opencircuit state, where the current through the solar cell is substantiallyzero, and the communication is based on a digital M-ary modulationscheme.
 15. The method of claim 11, wherein the voltage across the solarcell is continuously adjusted between the short circuit state, where thevoltage across the solar cell is substantially zero, and the opencircuit state, where the current through the solar cell is substantiallyzero, and the communication is based on an analog modulation scheme. 16.The method of claim 11, wherein the energy-harvesting system is based ona power converter selected from the group consisting of a boost DC-DCconverter, a buck-boost DC-DC converter, a buck DC-DC converter, and acombination thereof.
 17. The method of claim 13, wherein power of theemitted luminescence vs. voltage across the solar cell is about 0.12 μWat about 0 V for the SC state, about 0.25 μW at about 0.92 volt for theMPP state, and about 4.8 μW at about 1.02 V for the OC state, wherein“about” is defined as a degree of variability in a value or range within±10% of an associated value.
 18. The method of claim 11, furthercomprising receiving the solar cell-transmitted data by a receiver. 19.The method of claim 18, further comprising: receive wirelesslytransmitted data in form of incident light modulation to the solar cell;the solar cell receiving the incident light; and decoding dataassociated with the receiver-transmitted data.
 20. The method of claim19, wherein the energy-harvesting circuit is configured to harvestenergy from the solar during wireless communication from the receiver.